Method for providing metallic carbide coatings on graphite

ABSTRACT

Metallic or nonmetallic carbide coatings of substantially uniform thickness are provided on internal surfaces of elongated bores within a graphite substrate by vapor deposition. The coating process is achieved by the thermal decomposition of a selected halide and the inter-reaction with a hydrocarbon gas at temperatures of 900*-1200* C. at atmospheric pressure. A temperature gradient along the length of the bore is used to control the coating deposition rate for providing a carbide coating of substantially uniform thickness.

Napier et al.

[ Dec. 25, 1973 METHOD FOR PROVIDING METALLIC CARBIDE COATINGS ONGRAPHITE Inventors: John M. Napier, 113 Dartmouth Cir.; Anthony J.Caputo, 112 Caldwell Dr., both of Oak Ridge,

Tenn. 37830 [22] Filed: Jan. 12, 1971 [21] Appl. No.: 116,703

[52] US. Cl. 117/95, 117/97, 117/106 C, 117/107.2 R, 176/88 [51] Int.Cl. C23c 11/08, C23c 13/04 [58] Field of Search 117/95, 97, 106 C,117/107.2

[56] References Cited UNITED STATES PATENTS 3,501,356 3/1970 Li Chu117/106 C X 3,529,988 9/1970 Woemer 117/106 C X 3,160,517 12/1964 Jenkin117/107.2 R X 3,359,098 12/1967 Teaford... 117/l07.2 K X 3,464,8439/1969 Basche l17/I07.2 R X 3,420,707 1/1969 Hanak ll7/107.2 R X3,366,464 1/1968 Guichet et al. 117/106 C X 3,421,953 l/1969 McGuire eta1 117/97 3,400,016 9/1968 Enstrom et a1 Il7/lO7.2 R X OTHERPUBLICATIONS Powell, Oxley & Blocher, Vapor Deposition, 1966, pages307-310 and pages 328-330.

Primary Examiner-Carl D. Quarforth Assistant Examiner-Roger S. GaitherAttorney-John A. Horan [5 7] ABSTRACT Metallic or nonmetallic carbidecoatings of substantially uniform thickness are provided on internalsurfaces of elongated bores within a graphite substrate by vapordeposition. The coating process is achieved by the thermal decompositionof a selected halide and the inter-reaction with a hydrocarbon gas attemperatures of 900l200C. at atmospheric pressure. A temperaturegradient along the length of the bore is used to control the coatingdeposition rate for providing a carbide coating of substantially uniformthickness.

1 Claim, N0 Drawings METHOD FOR PROVIDING METALLIC CARBIDE COATINGS ONGRAPHITE The present invention relates generally to an improved methodof coating graphite with a metallic or nonmetallic carbide by vapordeposition, and more particularly to providing bores within a graphitesubstrate with a metallic carbide coating of substantially uniformthickness. This invention was made in the course of, or under, acontract with the U. S. Atomic Energy Commission.

Graphite is a well known high temperature material which possesses manyattractive properties that make it particularly suitable for use as anuclear reactor fuel matrix material. On the other hand, graphite reactsvery vigorously with hot hydrogen gas so as to present a seriousdrawback when attempting to use graphite as the fuel matrix material fornuclear rocket reactors since the hot hydrogen used as the propellant isheated as it passes through long bores or passageways in the fuelelement. In order to overcome this drawback graphite surfaces exposed tothe hot hydrogen are clad or coated with a protective carbide coatingwhich effectively inhibits or reduces the hydrogen attack.

Several methods of coating the inside walls of the long bores in thegraphite fuel matrix with carbides of niobium, tantalum, silicon,zirconium, tungsten and molybdenum have been considered. Vapordeposition has proven to be the most satisfactory technique forproviding continuous adherent coatings even though considerableattention must be exercised to avoid nonuniform coatings. Whileadjustment of the vapor flow pattern has been found to be a practicalapproach for uniformly coating exposed surfaces some difficulties areencountered when attempting to coat the inside of elongated smalldiameter bores. Obtaining a uniformly thick coating presented aconsiderable problem in that if the coating within the bores was formedin the usual manner by the thermal decomposition or hydrogen reductionof a metal halide-hydrocarbon mixture the coating was somewhat thickerat one end of the bore than at the other. The reason for the coatingbeing of nonuniform thickness is due to the temperature of the substratewhich, if at a uniform temperature above the reaction temperature, willcause the deposit to occur primarily at initial points of contact withthe substrate and with the coating thickness decreasing with increasinglength of the bore.

Previous efforts to provide uniform coatings inside elongated tubesinclude such techniques as described in U. S. Pat. No. 3,031,338, issuedApr. 24, 1962, which employs a movable heat zone positioned about thetube for heating selected areas of the tube as the heat zone is movdtherealong. In U. S. Pat. No. 3,318,724, issued May 9, 1967, there isdescribed a technique for providing uniformly thick tungsten tubing byemploying a furnace in which the entire furnace is above the reactiontemperature and then selectively and progressively cooled to below thereaction temperature to form the tubing. While previously knowntechniques such as exemplified by those described in the above patentshave demonstrated some success they have some shortcomings which detractfrom their usefulness in coating internal bores. For example, whenemploying a movable heating zone the internal coating over the length ofthe bore is somewhat irregular due to the variations in densityof thesubstrate and the specific mixture employed in the reaction, whichfrequently varies during the reaction in response to slight differencesin temperatures as would occur in the case of a moving heating zone. Theuse ofa heating arrangement wherein a selective and progressivecool-down of the substrate is utilized to produce tungsten tubing cannotbe satisfactorily used to coat internal bores since the selectivecool-down of the substrate can not be achieved with sufficient rapiditydue to the relatively large mass of the substrate so as to provide auniform coating. If the coating gases are passed over a coated surfaceat temperatures less than the temerature required to deposit thecarbide, the deposited coating will be removed or grossly attacked.Also, if a carbide coating is deposited at a first temperature, e.g.,l,l00C., and a second layer is required, the coating temperature for thesecond pass must be at least equal to or slightly greater than theoriginal temperature so as to insure that no damage will occur to theoriginal coating. With a movable heating zone several passes wouldlikely be necessary to provide a coating of the desired thickness.Further, since the deposition rate at any temperature in the reactionrange is small (about 1 mil per 12 hours is not uncommon), the movablezone heater would have to remain at a particular location for some timeduring any given pass to get the right thickness. Thus, the overall timerequired to coat bores 52 inches long would be exceptionally andundesirably long.

It is the principal objective or primary aim of the present invention toprovide an improved method for coating the inside of long bores ingraphite bodies with a continuous, adherent carbide coating ofsubstantially uniform thickness. The coating is nrovided by employing alow-temperature, atmospheric-pressure vapor deposition technique inwhich the selected carbide is deposited onto the heated graphitesubstrate by the thermal decomposition of a selected halide and theinterreaction of the latter with a hydrocarbon gas. The substantialuniformity in the thickness of the coating deposited on the graphitesurface portions defining the bores is achieved by using a temperaturegradient along the length of the bores ranging from a temperature abovethat at which thermal decomposition occurs at the entrance to the bores,i.e., the location in the bores initially contacted by the coatingvapor, to a substantially higher temperature at the far end of thebores.

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative method about to be described, or willbe indicated in the appended claims, and various advantages not referredto herein will occur to one skilled in the art upon employment of theinvention in practice.

Generally, the present invention is directed to a method of providing acarbide coating on the inside surface area of elongated small bores(0.109 inches or less in diameter by 52 inches long) located ingraphiteuranium dicarbide rods used as the fuel elements for nuclearrocket reactors. Each rod has 19 bores with a total surface area of 113inches or 16.48 inches for each bore. While the present inventionprimarily relates to the coating of such bores in the aforementionedfuel elements it will appear clear that the subject method can be easilyused for coating bores of various diameters and lengths in other metaland nonmetal substrates. Also, while the description below is directedto the fonnation of niobium carbide and zirconium carbide coatings, itis to be understood that the coatings can be formed of other metalliccarbides and nonmetallie carbides without departing from the spirit andscope of the present invention once a person skilled in the art hasderived the benefit of the teachings herein. For example, the carbidecoatings may be produced from halides selected from the group consistingof niobium halide, tantalum halide, hafnium halide, titanium halide,zirconium halide and silicon halide at temperatures ranging from 900C.l200C. at atmospheric pressure. The halide may be in either the form ofachloride or bromide but other vaporizable salts such as fluorides, oriodides, may be suitably used.

In order to provide the graphite surface portions or walls defining thebores with a carbide coating of substantially uniform thickness aniobium or zirconium halide is thermally decomposed and theninterreacted with a hydrocarbon gas in the presence of hydrogen. Thedecomposition of the halide and the reaction of resulting metal with thegaseous hydrocarbon occurs contiguous to the graphite substrate when thelatter is heated to a temperature at least as great as that required toeffect and sustain the thermal decomposition of the selected halide.During the coating operation the halide vapor is contacted with hydrogenand a suitable hydrocarbon gas and then introduced into a furnace orreaction vessel containing the graphite substrate. Preferably, theselected halide is heated in a solid form, e.g., salt, to provide thevapor which is then contacted by a stream of inert gas, e.g., helium orargon, used as a carrier for conveying the vapor into the furnace andfor controlling the partial pressure of the reactants during the flowthereof. During the conveyance of the reactants the temperature of thelatter is maintained at an elevation sufficient to keep the halide invapor form. Normally, a temperature of about 300C. is sufficient for thecompounds employed in the present invention. During the flow of thevapor to the furnace a stream of hydrogen and a hydrocarbon gas isco-mingled with the halide vapor-argon mixture. Upon entering thefurnace the vapor is introduced into a suitable preheater and manifoldsystem so as to preheat the vapor to the desired reaction temperaturebefore the vapor contacts the substrate and to uniformly distribute thevapor to the plurality of bores in the substrate. The preheater may beheated in the same manner as the substrate as will be discussed below.To aid in the activation of the carburizing reaction between the metalvapor and the hydrocarbon a gas such as HCl may be added to the mixture.

The hydrocarbon gas employed in the coating operation is preferablymethane, but other hydrocarbon gases such as C H C H and benzene thatcrack to form C H may be used in place of methane.

The rates at which the halide vapor, argon, methane, HCl, and hydorgenare brought into contact with the graphite substrate and providesatisfactory results may be selected from fairly wide ranges as follows:

Velocity in liters/min (l/m) Gas NbC ZrC Argon 0.24 to 0.48 0.l3 to 0.24HCl 0.04 to 0.08

Methane 0.00l to 0.002 0.001 to 0.002 Hydrogen 0.004 to 0.008 0.004 to0.l2 Nl'xCl salt 0.004 to 0.008

ZrCl salt 0.003 to 0.004

'Liters/min/element bore (-0.l09 in. diameter X 52 in. long).

The flow rates or quantity of argon used to form the of the methane andhydrogen control the properties of the carbide coating and thedeposition rate, respectively. The hydrogen is used to regulate the rateof reaction between the methane and the metal vapor. A hydrogen-to-NbClratio of about 3 to l is about maximum while a hydrogen-to-ZrCL, ratioof up to 40 to 1 may be used without experiencing difficulty.

The coating operation is carried out at atmopheric pressure which isadvantageous because of the difficulties encountered in coating bores ofthe length described herein by employing known vacuum techniques. Also,the equipment required for vacuum operation is subject to air inleakagewhich will prevent or inhibit the formation of carbide coating due toformation of oxides.

The surface temperature of the graphite substrate is critical to thecontrol of the deposition rate and the coating thickness of the carbidecoatings within the bore. The graphite substrate is heated in such amanner as to form a temperature gradient along the length of thegraphite bores whereby the temperature increases with distance from theends of the bores where the reactant vapors enter. Since the depositionrate is a function of temperature and gas composition the uniformity ofthe coating is controlled by providing a selected temperature gradientalong the elongated bore and employing a constant gas composition at theinlet of the bore. The temperature profiles along the length of the borebeing coated are set forth in Table 1 shown below. This table isrepresentative of niobium and zirconium carbide coatings but thetemperature profiles obtained from the other carbides are easilyachieved by experimentation.

TABLE 1 Point of Measurement (distance from end of the bore initiallyMeasurement made in the gas exit chamber 3 in. downstream from end ofthe bore.

The temperatures utilized in the coating operation are in a range from900 to 1,200C. with the maximum controllable temperature for reaction ofthe present invention being 1,200C. since above this temperature thedeposition rate is exceptionally fast and virtually all of the methaneis stripped from the gas stream. While the methane content may beincreased to slow down this stripping action the metallic carbidethickness along the length of the substrate becomes much more difficultto control because of the higher hydrocarbon content. The temperatureprofiles set forth for the carbides in Table 1 are near optimumtemperature gradients from the reactant gas formulations described aboveand may not vary more than il0C. at each of the noted control pointswithout excessively detracting from the uniformity of the coating.

The heating of the graphite substrate to provide the desired temperaturegradient along the length of the bores may be accomplished by encirclingthe substrate with a plurality of induction heating coils axially spacedapart along the length of the bore with each coil providing preselectedsubstrate temperatures at desired locations. However, any suitablecommercially available heating system capable of providing the necessarytemperature gradient along the length of the bore in the substrate maybe used.

In order to provide a more facile understanding of the method of thepresent invention examples of typical niobium carbide and zirconiumcarbide coating operations are set forth below. In these examples thecarbide coatings were provided on the walls of bores through a graphitesubstrate with the bores each having a diameter of approximately 0.109inch and a length of 52 inches. Also, the bracketed figures relate tovolume percent provided by each ingredient of the coating mixture.

EXAMPLE I A niobium carbide coating was formed at atmospheric pressureon the walls of the bores in the graphite substrate by vaporizing NbCl.salt (1.62 percent) by heating it to a temperature of 300C. in avaporizer; passing argon over the salt to eject the NbCl vapors from thevaporizer into a conduit heated to 300C.; introducing into the conduit ametered gaseous mixture of argon (total argon content 82.72 percent),hydrogen (1.51 percent), HCl (13.78 percent), and methane (0.37percent); conveying the reactant vapor by means of the flowing argoninto the reaction vessel where the vapor was preheated to 1,040C. andthen brought into contact with the graphite substrate heated totemperatures of 1,040C., 1,095C., 1120C., and 1145C. at distances orlocations of 3 inches, 29 inches, 43 inches and 55 inches (lasttemperature point is located in gas exit chamber, 3 inches fromend ofsubstrate), respectively, from the end of the substrate initiallycontacted by the vapor; maintaining the reactant flow and substratetemperature for a duration of 21 hours during which the thermaldecomposition of the NbCl vapor and the inter-reaction of the resultingmetal and methane occurred to provide a niobium carbide coating ofthicknesses set forth in Table 2 below.

TABLE 2 Point of Measurement Thickness of Coating (mils) A zirconiumcarbide coating was provided on another graphite substrate by employingthe procedure set forth in Example I and a reactant vapor of argon(95.74 percent), vaporized ZrCl, (1.91 percent), methane (0.43 percent),and hydrogen (1.91 percent). The temperature profile used was 1,055C.,l,l40C., 1.175C., and 1,200C. at locations along the bore correspondingto those set forth in Example I. The reactant flow and substratetemperature was maintained for 50 hours and provided a zirconium carbidecoating of thicknesses set forth in Table 2 above.

It will be seen that the present invention provides a desirable approachto providing carbide coatings on complex substrates and is particularlysuitable for coating passageways or bores through graphite articlessince the tempratures and pressures are easily attained without theexercise of time-consuming and complex mechanisms.

What is claimed is:

1. A method of providing a niobium carbide or zirconium carbide coatingof substantially uniform thickness on surface portions of a graphitesubstrate that define at least one elongated passageway extendingthrough the substrate, comprising the steps of contacting said surfaceportions of the graphite substrate with a gaseous mixture of a volatilehalide selected from the group consisting of niobium pentachloride andzirconium tetrachlon'de, argon, hydrogen, hydrogen chloride when thevolatile halide is niobium pentachloride, and methane, and heating thesubstrate to provide an increasing gradient of temperatures from alocation on said surface portions initially contacted by the gaseousmixture to another location on said surface portions at the opposite endof said at least one passageway with said gradient of temperatures beingin a range of 900C. to 1,200C. for thermally decomposing the volatilehalide and effecting a reaction of the decomposed halide with thehydrogen and methane for depositing the carbide coating on said surfaceportions, said gradient of temperatures providing sufficient temperatureincreases from the first mentioned location through said anotherlocation to effect a substantially uniform deposition rate of saidcoating on the surface portions defining said at least one passageway,said gaseous mixture being at atmospheric pressure with each gas in themixture when the volatile halide is niobium pentachloride having a flowrate in the range of 0.004 to 0.008 l/m for the niobium pentachloride,0.001 to 0.002 l/m for the methane, 0.004 to 0.008 l/m for the hydrogen,0.24 to 0.48 l/m for the argon, and 0.004 to 0.008 l/m for the hydrogenchloride, and when the volatile halide is zirconium tetrachloride eachgas in the mixture having a flow rate in the range of 0.003 to 0.004 l/mfor the zirconium tetrachloride, 0.001 to 0.002 l/m for the methane,0.004 to 0.12 l/m for the hydrogen, and 0.13 to 0.24 l/m for the argon.

Patent No. 8 Dated Inventor(s) J. M. Napier, and A. J, Caputo It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

after [76]- add:

"Z731 Assignee: The United States of America as represented by theUnited States Atomic Energy Commission Signed and sealed this 21st dayof May 197M (SEAL) fattest:

LDF-IAFLD i'l .FLETLII-ILIR, JR. b MARSHALL DAL-W Attesting OfficerLlommissio'ner of Patents F ORM PO-IOSD (10-69) USCOMM-DC 60376-P69 1:us GOVERNMENT PRINTING OFFICE: I999 0-366-33A,

Patent No. q g qq Dated m: 25 13213 Inventor(s) J. M. Napier, and A. J,Caputo It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

after [76]- add:

"173] Assignee: I'he United States of America as represeated by theUnited States Atomic Energy Commission Signed and sealed this 21st dayof May 197 (SEAL) Attest: v

EDWARD LLFUB'ICIMR IPA I U. IMRSHALL DAWN Attesting OfficerCommissionerof Patents FORM PO-1050 (10-69) USCOMM-DC scam-P69 1 E US.GOVERNMENT PRINTING OFFICE I989 0-366-334,

